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ELM329
ELM329DSC
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AT Command Descriptions (continued)
is always as set by AT ST), while AT2 is a more
aggressive version of AT1 (the effect is more
noticeable for very slow connections – you may not
see much difference with faster OBD systems). The
J1939 protocol does not support Adaptive Timing – it
uses fixed timeouts as set in the standard.
BD
[ perform an OBD Buffer Dump ]
All messages sent and received by the ELM329
are stored temporarily in a set of twelve memory
storage locations called the OBD Buffer. Occasionally,
it may be useful to see the contents of this buffer,
perhaps to see why a request failed, to see the header
bytes in the last message, or just to learn more of the
structure of OBD messages. You can ask at any time
for the contents of this buffer to be ‘dumped’
(ie printed). When you do, the ELM329 sends a length
byte (representing the length of the current message in
the buffer) followed by the contents of all twelve OBD
buffer locations. For example, here’s one ‘dump’:
>AT BD
0C
00
00
07
E8
03
41
05
42
00
00
00
00
The 0C is the length byte - it tells us that the
following 12 bytes are valid. The actual bytes that have
been sent or received appear after the length. Note
that wakeup (CAN periodic) messages do not use the
buffer as an intermediate step, so you are not able to
see them with AT BD.
BI
[ Bypass the Initialization sequence ]
This command should be used with caution. It
allows the currently selected protocol to be made
active without requiring any sort of initiation or
handshaking to occur. The initiation process is
normally used to validate the protocol, and without it,
results may be difficult to predict. It should not be used
for routine OBD use, and has only been provided to
allow the construction of ECU simulators and training
demonstrators.
BRD hh
[ try Baud Rate Divisor hh ]
This command is used to change the RS232 baud
rate divisor to the hex value provided by hh, while
under computer control. It is not intended for casual
experimenting - if you wish to change the baud rate
from a terminal program, you should use PP 0C.
Since some interface circuits are not able to be
operated at high data rates, the BRD command uses a
sequence of sends and receives to test the interface,
with any failure resulting in a fallback to the previous
baud rate. This allows several baud rates to be tested
and a reliable one chosen for the communications.
The entire process is described in detail in the ‘Using
Higher RS232 Baud Rates’ section, on pages 45 and
46.
If successful, the actual baud rate (in kbps) will be
4000 divided by the divisor (hh).
BRT hh
[ set Baud Rate Timeout to hh ]
This command allows the timeout used for the
Baud Rate handshake (ie. AT BRD) to be varied. The
time delay is given by hh x 5.0 msec, where hh is a
hexadecimal value. The default value for this setting is
0F, providing a 75 msec timeout. Note that a value of
00 does not result in 0 msec - it provides the
maximum time of 256 x 5.0 msec, or 1.28 seconds.
C0 and C1
[ Control output off* or on ]
These commands are used to set the level at the
Control output (pin 4). The AT C0 command sets it to a
low logic level (0V), while AT C1 sets it to a high level
(5V). After a system reset or wakeup from low power
mode (unless PP 0F bit 0 = ‘1’), the Control output will
be reset to a low level.
CA
[is there CAN Activity at pin 11?]
This command is used to determine if there is a
CAN signal present at pin 11 (the CAN Monitor pin). If
there is, the response will be the letter ‘Y’ (for yes),
while if there is no signal, the response will be the
letter ‘N’, for no. If there has been no signal detected
since the last reset, the output will be a dash (‘-’).
CAF0 and CAF1
[ CAN Auto Formatting off or on ]
These commands determine whether the ELM329
assists you with the formatting of the CAN data that is
sent and received. With CAN Automatic Formatting
enabled (CAF1), the IC will automatically generate the
formatting (PCI) bytes for you when sending, and will
remove them when receiving. This means that you can
continue to issue OBD requests (01 00, etc.), without
regard to the extra bytes that some CAN systems
require. Also, with formatting on, any extra (unused)